Fracture Behavior of Colloidal Polymer Particles in Fast-Frozen Suspensions Viewed by Cryo-SEM

Abstract

Cryogenic scanning electron microscopy (cryo-SEM) is a powerful technique of visualizing the state of microstructure, or nanostructure, of colloidal polymer suspensions, or dispersions, after they have been immobilized by fast-freezing and fractured for imaging. The fracture is done at −196 °C, the normal boiling point of liquid nitrogen, which is far below the glass transition temperature of both bulk and fully coalesced particles of the polymers examined. The cryo-SEM images show a range of responses of particles to the fracture that propagated past them through ice: cleanly sliced through, i.e., broken, partially drawn and unbroken, drawn to breakage, and neither drawn nor broken. The drawn particles often display curious features called "pullouts" that have been generated by plastic deformation of parts of the particles that remain in a fracture surface, or sometimes both surfaces near the tip of a fracture that halted. The main morphologies of pullouts are mushroom shaped, spool shaped, and awl shaped. Pullouts form when the molecular weight (Mw) of the polymer exceeds twice the entanglement molecular weight (Me); they do not form when cross-linking density exceeds a certain level. We hypothesize that nonbulk molecular organization of submicron colloidal particles, perhaps most disturbed in a surface zone, promotes the formation of pullouts. Pullouts have been seen in particles from 500 nm in diameter, the largest examined, down to 30 nm, the smallest. At least when Mw is greater than twice Me, Tg does not affect the formation of pullouts. The pullout features indicate that the yield behavior, and thus the mechanical properties, of submicron particles synthesized by emulsion polymerization differs from that of polymer crazing in bulk and in thin films; the molecular-level explanation stands as a challenge to macromolecular science.